1. Field of the Invention
The present invention relates to an optical pick-up head for reading information recorded in an optical record medium such as an opto-magnetic record medium comprising a light source for emitting an incident light beam, a beam splitter for directing the incident light beam emitted by said light source onto an optical record medium and for directing a return light beam reflected by said optical record medium in a direction which is different from a direction directing the light source, a focusing lens for focusing the return light beam emanating from said beam splitter into a converged light flux, a polarizing beam splitter arranged in said converged light flux for separating said converged light beam into P-polarized light beam and S-polarized light beam, and a photodetector including first and second light receiving elements for receiving said P-polarized light beam and S-polarized light beam, respectively.
2. Description of the Prior Art
There have been proposed various kinds of optical pick-up heads. In particular, an optical pick-up head for use in an opto-magnetic disk has been disclosed in Japanese Patent Application Laid-open Publication Kokai Hei 2-118936, in which a linearly polarized light beam reflected from the opto-magnetic disk is made incident upon a polarizing beam splitter and is divided thereby into P-polarized beam and S-polarized beam, then the P- and S-polarized beams are separately received by light receiving elements of a photodetector, and a difference between outputs of these light receiving elements is derived as a reproduced signal. It should be noted that the P-polarized beam is a polarized beam whose polarizing direction is parallel with an incident plane and the S-polarized beam is a polarized beam whose polarizing direction is perpendicular to the incident plane, so that the polarizing directions of these polarized beams are perpendicular to each other.
FIG. 1 is a schematic view showing a general construction of the known optical pick-up head disclosed in the above mentioned Kokai Hei 2-118936. A linearly polarized beam emitted by a laser light source 1 is converted into a parallel laser beam by means of a collimator lens 2 and is made incident upon a beam splitter 3. A laser beam transmitted through the beam splitter 3 is made incident upon an objective lens 4 and then is made incident upon an opto-magnetic record medium 5 as a very fine spot. A return laser beam reflected by the opto-magnetic record medium 5 is made incident upon the beam splitter 3 by means of the objective lens 4. The return laser beam reflected by the beam splitter 3 is then focused by a converging lens 6 and is made incident upon a polarizing beam splitter 7, and is divided into P-polarized beam and S-polarized beam. As is well known in the art, the polarizing direction of the linearly polarized laser beam projected onto the opto-magnetic record medium 5 is rotated in accordance with content of the information recorded on the record medium, so that intensities of the P-polarized and S-polarized beams are changed in accordance with the content of the information. These P- and S-polarized beams are separately received by light receiving elements 8a and 8b of a photodetector 8. Then, output signals from these light receiving elements 8a and 8b are supplied to differential amplifier 9 to derive a difference therebetween. An output signal of the differential amplifier 9 represents a difference in intensity between the P- and S- polarized beams. In this manner, the information recorded on the opto-magnetic record medium 5 can be reproduced by suitably processing the output signal of the differential amplifier 9.
FIG. 2 is a schematic view illustrating the detailed construction of the polarizing beam splitter 7 shown in FIG. 1. The polarizing beam splitter 7 comprises two prisms 10 and 11 which are cemented to each other by means of a suitable adhesive while a polarizing plane 7a is interposed therebetween. The polarizing plane 7a is formed by stacking a number of thin films 12 and 13 having higher and lower refractive indices alternately. In the known optical pick-up head shown in FIG. 1, the polarizing beam splitter 7 illustrated in FIG. 2 is arranged to be rotated by 45.degree. with respect to the polarizing direction of the converged light beam, so that the P-polarized beam transmits the polarizing plane and the S-polarized beam is reflected by the polarizing plane. In this manner, the P- and S-polarized light beams can be separated from each other.
It should be noted that on a surface 14 of the prism 11 is applied the polarizing coating which has a small dependency upon an incident angle, so that off-axis light rays of the converged light beam emanating from the single converging lens 6 are similarly separated into the P- and S-polarized beams. An outermost light ray is deviated from the optical axis by an angle .theta. and is made incident upon the polarizing plane 7a at an incident angle of .alpha.-.theta./n, wherein .alpha. is an incident angle of an on-axis ray and n is a refractive index of the prism 10. A deviation of the incident angle of the outermost light ray with respect to that of the on-axis light ray amounts usually 3.degree..
As shown in FIG. 2, the known polarizing beam splitter 7 is formed by two prisms 10 and 11, and the polarizing plane 7a is provided between the interface between these prisms. The polarizing coating is applied on the plane 14 of the prism 11, and on a surface 15 of the prism from which the P-polarized beam emanates there is applied an anti-reflection coating. On incident and exit surfaces 16 and 17 of the other prism 10 there are applied anti-reflection coatings, and on a side surface 18 there is applied a protection coating. Further, the two prisms 10 and 11 are cemented with each other by means of a cementing agent. Therefore, when the known polarizing beam splitter 7 is manufactured, after applying the required coatings on the above mentioned five surfaces of the prisms 10 and 11 and forming the polarizing plane 7a on the surface 14, these prisms are cemented. This results in that the manufacturing process is rather complicated and the cost of the polarizing beam splitter is liable to be increased. The manufacturing cost would be further increased due to the fact that the known polarizing beam splitter 7 requires the two prisms, i.e. parallelogram prism 10 and triangular prism 11, so that the polarizing beam splitter is liable to be further expensive.
Moreover, when the thin film 12 having a high refractive index is made of TiO.sub.2, the TiO.sub.2 film reacts with the cementing agent, and a light absorption occurs due to a chemical reaction. Therefore, even if a transmissivity of the P-polarized beam is designed to be 100%, in practice the transmissivity is decreased lower than 95%. In this manner, the known polarizing beam splitter suffers from a loss in an amount of information light.